The invention relates to a process for air separation by rectification, in which in a first rectification stage air is preliminarily separated into a nitrogen-rich and an oxygen-rich fraction. The two fractions are fed to a second rectification stage and separated into oxygen and nitrogen. A stream containing essentially oxygen and argon is removed from the second rectification stage at an intermediate point and by rectification in a raw argon column is separated into an argon-rich fraction and a liquid fraction containing essentially oxygen. The fraction containing essentially oxygen is fed back into the second rectification stage. The invention further relates to an apparatus for carrying out such a process.
Such a process, in which, besides oxygen and nitrogen, a raw argon fraction is also recovered, is described in U.S. Pat. No. 4,575,388. Argon, whose boiling point is between the boiling points of nitrogen and oxygen, is present at an increased concentration at an intermediate point of the second rectification stage. At this point, a fraction is removed which is enriched with argon and contains mainly oxygen as well as certain portions of nitrogen. By rectification, the argon-enriched fraction is separated in the raw argon column into a raw argon fraction, which is polluted by residual oxygen and nitrogen, and a liquid bottom fraction consisting essentially of oxygen. The bottom fraction is fed back into the second rectification stage.
Besides raw argon, oxygen is recovered as another product in this process. Oxygen is removed at the lower end of the second rectification stage.
The oxygen-rich liquid, which accumulates in the bottom of the raw argon column, exhibits a relatively high concentration of impurities. The enriched argon fraction from the second rectification stage, besides oxygen and nitrogen, also contains the impurities krypton, xenon and hydrocarbons, all of which accumulate in the bottom of the raw argon column. The impurities reach the bottom of the second rectification stage by the delivery of bottom liquid from the argon column into the second rectification stage and thus into the oxygen removed as a separation product.
Because of the impurities in the oxygen, the process does not permit the recovery of high-purity product streams--free of krypton, xenon and hydrocarbons--from the second rectification stage, especially product liquid oxygen. High-purity oxygen is necessary, for example, as breathable oxygen and also in the electronic industry.
In addition, the product nitrogen recovered in the known process contains traces of other gases, for example, helium, neon, hydrogen and carbon monoxide. Such nitrogen is insufficient for the modern semiconductor industry where nitrogen of the highest purity is required.
Carbon monoxide can be removed catalytically. Some helium, neon and hydrogen can be removed by a helium outlet, usually placed at the head of the first rectification stage; however, the helium outlet results in only a slight reduction of these impurities.